This invention relates to pilot controlled spool valves and, in its presently preferred embodiments, to new and improved servo controlled pilot stages and spool valves.
Spool-type valves are typically used to control the flow of pressurized fluid, such as hydraulic oil, water or air to a hydraulic cylinder or similar device. The size and diameter of the spool determine the flow capacity of the valve and thus the rate of energy transfer through the valve. The position of the spool within its valve body controls the amount and direction of fluid flow through the valve. Because the fluid flow forces and spool mass are typically high, pilot stages can be used to control the spool position which in turn controls the fluid flow.
There are generally three types of spool-type valves: directional control, proportional control, and servo or feedback control. Directional control valves are used to commence and interrupt fluid flow. These valves are used in the majority of fluid control applications. Proportional control pilot valves control the amount of fluid flow in proportion to an input signal. The use of these valves in applications is increasing. Servo-type pilot valves use mechanical feedback from the spool to a pilot stage to control spool position. These valves are used in high performance, proportional control applications where accurate closed loop control is essential.
One conventional type of directional spool valve uses a solenoid to control spool position. A first solenoid is attached to one end of the valve housing and the solenoid plunger is coupled to one end of the valve spool. A second solenoid is attached to the opposite end of the valve housing and the plunger of the second solenoid is coupled to the opposite end of the valve spool. The solenoids are alternately energized to move the spool and start and stop the fluid flow. Specifically, when the first solenoid is energized, it translates its plunger and drives the spool in one direction to turn on fluid flow in one direction. When the second solenoid is energized, its plunger drives the spool in the opposite direction to turn on fluid flow in the other direction.
This type of conventional directional valve has several drawbacks. The two solenoids and their associated electrical connections add bulk, weight, and significant power consumption to the valve package. The plungers are fabricated from iron and are relatively heavy and thus require significant electrical energy to be translated by the solenoid coils. Twenty-four volts and one ampere are typical electrical requirements of such solenoids which thus consume twenty-four watts of power. Of greater concern in systems is the slow response of such solenoids which is generally about 100 milliseconds.
One type of conventional proportional valve also uses a solenoid to control spool position. In this valve, however, a solenoid and its plunger are attached to only one end of the spool. A spring is attached to the other end of the spool. When the solenoid is energized, it translates the plunger which pushes the spool against the bias of the spring. The force of the spring provides proportional control of the flow of fluid. When the solenoid is de-energized, the spring returns the spool to the off position.
This type of proportional valve also shares the drawbacks of the solenoid controlled directional control valve. Specifically, high electrical current, and thus power, is necessary to move the spool against the spring. Moreover, this design is not well suited for high pressure applications.
A widely used servo-type valve is disclosed in U.S. Pat. No. 3,023,782 (Chaves). This valve uses a torque motor pilot stage with negative feedback provided by a flapper 73 in mechanical contact with the spool. The pilot stage shifts the spool, which can be subject to large fluid forces, in response to a small electrical signal to the torque motor. The position of the flapper is negatively fed back to the pilot stage to control the spool position. This negative feedback provides linearity and minimizes hysteresis.
While the Chaves servo valve provides some advantages, it also has significant disadvantages. These valves are complex and expensive to manufacture. The current price for a 10 gallon per minute valve is around $1000.00. Furthermore, these valves are susceptible to clogging due to the small orifices (on the order of 0.005 inch) in the pilot stage. Thus, extensive filtering of hydraulic fluid, is necessary to avoid contamination problems.
Another servo operated spool valve is disclosed in U.S. Pat. No. 3,106,224 (Moss). This patent discloses a spool 1 and a cylindrical spindle 13, which extends through an axial bore in the spool and the two ends of the valve housing 7. Two helical grooves 15 and 16 are formed in the surface of the spindle and are spaced from each other by approximately one-half helical pitch, so that each groove extends from one end of the valve housing cavity past a pair of diametrically opposed radial bores 17 in the spool. In its central position, the radial bores should be inside the central port 3 of the valve housing, and each groove should uncover equal parts of one of the radial bores.
The spool is maintained in its central position by a continuous flow of oil through the valve housing and spool that provides equal fluid pressure at both ends of the housing bore. In particular, the oil flows along two branches from the pressure inlet 24, through ports 2 and 4, passages 20 and 22, and orifices 21 and 23, to the two end chambers of the bore in housing 7, through the grooves 15 and 16 and the radial bores 17 to the drain port 12. In this central, null position, lands 5 and 6 block the flow of oil through the service ports 10 and 11.
In order to move the spool axially, the spindle 13 is rotated. This will cause one groove to uncover a greater portion of one radial bore and the other groove to cover a greater portion of the opposite radial bore. As a result, the fluid pressure in one end chamber of the valve housing will be greater than the other, and the spool will move towards the chamber of lower pressure until the fluid pressure in each chamber is equal. At this point, each radial bore will be uncovered the same amount again. The axial movement of the spool is proportional to the rotary displacement of the spindle 13.
The Moss servo valve is less complex and more desirable than the Chaves servo valve. However, the Moss servo valve also has some significant drawbacks. The Moss valve is designed to have continuous oil flow, even at null, between both ends of the valve housing to balance the pressure across the spool. This continuous flow requirement consumes power even at null. The design complicates the manufacture of the valve. The spindle grooves 15 and 16 and radial bores 17 must be designed in a relationship that facilitates constant flow. The passages 20 and 22 and orifices 21 and 23 must be machined into the outer lands 18 and 21 of the spool. The orifices 21 and 23 must be the same size so that each end chamber has about one-half of the fluid pressure at the null position. The orifices and radial holes should also be small to minimize flow at null. The small holes, however, are more prone to contamination.